The present disclosure generally relates to ventilator systems and methods of control therefor, and more particularly to manual ventilation systems and methods providing fresh gas flow into the manual ventilation circuit.
Anesthesia machines are optimized for delivering anesthesia to a patient using volatile anesthetic agent liquids. In such systems, the anesthetic agent is vaporized and mixed into the breathing gas stream in a controlled manner to provide a gas mixture for anesthetizing the patient for a surgical operation. The most common volatile anesthetic agents are halogenated hydrocarbon chains, such as halothane, Enflurane, Isoflurane, Sevoflurane and Desflurane. Additionally, nitrous oxide (N2O) can be counted in this group of volatile anesthetic agents, although the high vapor pressure of nitrous oxide causes nitrous oxide to vaporize spontaneously in the high pressure gas cylinder, where it can be directly mixed with oxygen. The anesthetizing potency of nitrous oxide is seldom enough to anesthetize a patient and therefore is typically mixed with one or more other volatile agents.
Anesthesia machines designed to deliver volatile anesthetic agents are designed to provide oxygen to the patient and eliminate carbon dioxide, while preserving the anesthetic gases. These goals are typically met using a re-breathing circuit, where an exhaled gas is cleaned to remove carbon dioxide and then re-introduced into the inhalation limb leading to the patient. In such a re-breathing circuit, carbon dioxide is absorbed from the expired gases by a carbon dioxide absorber. Before inhalation by the patient, the inhalation gas is supplied with additional fresh gas, such as air and/or oxygen mixed with vaporized aesthetic agents.
In addition to mechanical ventilation provided by the anesthesia machine, anesthesia ventilation involves ability for the clinician to manually ventilate the patient. Manual ventilation is typically provided by the clinician squeezing and releasing a breathing bag which then forces circulation of gasses through the re-breathing circuit. This functionality is often utilized during an anesthesia induction and when weaning the patient from the anesthesia and ventilator to provide assistance with spontaneous breathing and lung recruitment. A desired property of manual ventilation system is given haptic feedback of the patient breath volume. Such feedback is achieved when the patient exhalation volume is collected to the manual breathing bag.
The currently most used arrangement in ventilating manually is to have a breathing system equipped with an APL (Airway Pressure Limiting) valve. The APL valve is set by the clinician to an APL setting—a pressure setting at which the valve opens to let gas escape from the re-breathing circuit. When the manual ventilation bag is squeezed the gas volume is initially delivered to the patient, but when the APL pressure limit is reached the valve starts to bleed gas out from the circuit. The valve will form a resistance and as the bag is further squeezed some of the volume will go to the patient and some will bleed through the valve.